Wafer-level fan-out package with enhanced performance

ABSTRACT

The present disclosure relates to a packaging process to enhance performance of a wafer-level package. The disclosed package includes multiple mold compounds, a multilayer redistribution structure, and a thinned die with a device layer and die bumps underneath the device layer. The multilayer redistribution structure includes package contacts at a bottom of the multilayer redistribution structure and redistribution interconnects connecting the die bumps to the package contacts. A first mold compound resides around the thinned die to encapsulate sidewalls of the thinned die, and extends beyond a top surface of the thinned die to define an opening over the thinned die. A second mold compound resides between the multilayer redistribution structure and the first mold compound to encapsulate a bottom surface of the device layer and each die bump. A third mold compound fills the opening and is in contact with the top surface of the thinned die.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 62/743,966, filed Oct. 10, 2018, the disclosure of which is hereby incorporated herein by reference in its entirety.

This application is related to U.S. patent application Ser. No. 16/454,687, filed Jun. 27, 2019, now published as U.S. patent application publication number 2020/0115220, entitled “WAFER-LEVEL FAN-OUT PACKAGE WITH ENHANCED PERFORMANCE;” the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a packaging process, and more particularly to a packaging process to provide a wafer-level fan-out (WLFO) package with enhanced thermal and electrical performance.

BACKGROUND

The wide utilization of cellular and wireless devices drives the rapid development of radio frequency (RF) technologies. The substrates on which RF devices are fabricated play an important role in achieving high level performance in the RF technologies. Fabrications of the RF devices on conventional silicon substrates may benefit from low cost of silicon materials, a large scale capacity of wafer production, well-established semiconductor design tools, and well-established semiconductor manufacturing techniques.

Despite the benefits of using conventional silicon substrates for RF device fabrication, it is well known in the industry that the conventional silicon substrates may have two undesirable properties for the RF devices: harmonic distortion and low resistivity values. Harmonic distortion is a critical impediment for achieving high level linearity in the RF devices built over silicon substrates. In addition, the low resistivity encountered in the silicon substrates may degrade quality factors (Q) at high frequencies of microelectromechanical systems (MEMS) or other passive components.

Further, high speed and high performance transistors are more densely integrated in RF devices, even as they are required to carry more power. Consequently, the amount of heat generated by the RF devices will increase significantly due to the large amount of power passing through the transistors, the large number of transistors integrated in the RF devices, and the high operation speed of the transistors. Accordingly, it is desirable to package the RF devices in a configuration for better heat dissipation.

Wafer-level fan-out (WLFO) packaging technology and embedded wafer-level ball grid array (EWLB) technology currently attract substantial attention in portable RF applications. WLFO and EWLB technologies are designed to provide high density input/output ports (I/O) as well as low profile package height without increasing the size of the component semiconductor chips. The I/O pad size on the chip remains small keeping die size to a minimum. This capability allows for densely packaging the RF devices within a single wafer.

To accommodate the increased heat generation of the RF devices, to reduce deleterious harmonic distortion and quality factor losses of the silicon substrate, and to utilize advantages of WLFO/EWLB packaging technologies, it is therefore an object of the present disclosure to provide a packaging process for a wafer-level fan-out (WLFO) package with enhanced thermal and electrical performance.

SUMMARY

The present disclosure relates to a packaging process to provide a wafer-level fan-out (WLFO) package with enhanced thermal and electrical performance. According to an exemplary process, a mold package that includes a first intact die, a first mold compound, and a second mold compound is provided. Herein, the first intact die includes a first device layer, a first dielectric layer over the first device layer, a first silicon substrate over the first dielectric layer, and a number of first die bumps underneath the first device layer. The first mold compound resides around the first intact die to encapsulate sidewalls of the first intact die, while a backside of the first silicon substrate is exposed. The second mold compound is formed underneath the first mold compound to cover a bottom surface of the first device layer and encapsulate each first die bump. Next, the first silicon substrate of the first intact die is substantially removed to provide a first thinned die and form an opening within the first mold compound and over the first thinned die. A top surface of the first thinned die is exposed at a bottom of the opening. A third mold compound is then applied to substantially fill the opening and directly contact the top surface of the first thinned die. After the third mold compound is formed, the second mold compound is thinned to expose each first die bump. At last, a multilayer redistribution structure is formed underneath the second mold compound. The multilayer redistribution structure includes a number of package contacts at a bottom of the multilayer redistribution structure and redistribution interconnections that connect the package contacts to certain ones of the first die bumps.

In one embodiment of the exemplary process, the multilayer redistribution structure is glass-free, and connections between the redistribution interconnections and each first die bump are solder-free.

In one embodiment of the exemplary process, the first intact die provides a microelectromechanical systems (MEMS) component.

In one embodiment of the exemplary process, the first intact die is formed from a silicon-on-insulator (SOI) structure. The first device layer of the first intact die is formed from a silicon layer of the SOI structure, the first dielectric layer of the first intact die is a buried oxide layer of the SOI structure, and the first silicon substrate of the first intact die is a silicon substrate of the SOI structure.

In one embodiment of the exemplary process, the mold package further includes a second intact die, which includes a second device layer, a second silicon substrate over the second device layer, and a number of second die bumps underneath the second device layer. The first intact die is taller than the second intact die. The first mold compound encapsulates sidewalls and a top surface of the second intact die. The second mold compound covers a bottom surface of the second device layer and encapsulates each second die bump. Herein, the first intact die provides a MEMS component and the second intact die provides a complementary metal-oxide-semiconductor (CMOS) controller that controls the MEMS component.

In one embodiment of the exemplary process, the third mold compound has an electrical resistivity greater that 1E6 Ohm-cm.

In one embodiment of the exemplary process, the third mold compound has a thermal conductivity greater than 2 W/m·K.

In one embodiment of the exemplary process, the third mold compound has a thermal conductivity greater than 10 W/m·K.

In one embodiment of the exemplary process, the first mold compound, the second mold compound, and the third mold compound are formed from different materials.

In one embodiment of the exemplary process, the second mold compound and the third mold compound are formed from a same material.

In one embodiment of the exemplary process, the top surface of the first thinned die exposed at the bottom of the opening is a top surface of the first dielectric layer.

In one embodiment of the exemplary process, the mold package is formed by providing a precursor package that includes the first intact die and the first mold compound. Herein, the first mold compound encapsulates the sidewalls and a top surface of the first intact die, and the bottom surface of the first device layer and each first die bump are exposed. Next, the second mold compound is formed underneath the first mold compound to cover the bottom surface of the first device layer and encapsulate each first die bump. The first mold compound is then thinned down to expose the backside of the first silicon substrate.

In one embodiment of the exemplary process, the precursor package is formed by attaching a first non-bump die to a top surface of a carrier. Herein, the first non-bump die includes the first device layer, the first dielectric layer over the first device layer, and the first silicon substrate over the first dielectric layer. Next, the first mold compound is applied over the top surface of the carrier to encapsulate the first non-bump die. The carrier is then removed to expose the bottom surface of the first device layer of the first non-bump die. At last, the number of the first die bumps are formed at the exposed bottom surface of the first device layer to complete the first intact die. The first die bumps are not covered by the first mold compound.

In one embodiment of the exemplary process, a portion of the third mold compound resides over the first mold compound.

In one embodiment of the exemplary process, a top surface of the first mold compound and a top surface of the third mold compound are coplanar.

According to another embodiment, the exemplary process further includes applying a fourth mold compound over the top surface of the first mold compound and the top surface of the third mold compound.

In one embodiment of the exemplary process, the first mold compound and the fourth mold compound are formed from a same material.

In one embodiment of the exemplary process, the third mold compound is applied by a compression molding process.

In one embodiment of the exemplary process, a molding pressure used for applying the third mold compound is lower than 1000 psi.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIGS. 1-4 illustrate exemplary steps to provide a precursor package according to one embodiment of the present disclosure.

FIG. 5 provides a flow diagram that illustrates an exemplary process to provide a wafer-level fan-out (WLFO) package with enhanced performance from the precursor package shown in FIG. 4.

FIGS. 6-13 illustrate the steps associated with the process of FIG. 5.

It will be understood that for clarity of illustration, FIGS. 1-13 may not be drawn to scale.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to a packaging process to provide a wafer-level fan-out (WLFO) package with enhanced thermal and electrical performance. FIGS. 1-4 illustrate exemplary steps to provide a precursor package according to one embodiment of the present disclosure. Although the exemplary steps are illustrated in a series, the exemplary steps are not necessarily order dependent. Some steps may be done in a different order than that presented. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated in FIGS. 1-4.

Initially, a first non-bump die 10N and a second non-bump die 12N are attached to a top surface of a carrier 14, as illustrated in FIG. 1. In different applications, there may be fewer or more dies attached to the carrier 14. For instance, in some applications, there may be multiple first non-bump dies 10N attached to the carrier 14, while the second non-bump die 12N is omitted. In some applications, there may be multiple first non-bump dies 10N and one second non-bump die 12N attached to the carrier 14. In some applications, there may be multiple first non-bump dies 10N and multiple second non-bump dies 12N attached to the carrier 14. In some applications, besides the first and second non-bump dies 10N and 12N, there may be some integrated passive device dies (not shown) attached to the carrier 14. To enhance the attachment, there may be an adhesive layer (not shown) at the top surface of the carrier 14.

The first non-bump die 10N includes a first device layer 16, a first dielectric layer 18 over the first device layer 16, and a first silicon substrate 20 over the first dielectric layer 18. As such, a bottom surface of the first device layer 16 is a bottom surface of the first non-bump die 10N, and a backside of the first silicon substrate 20 is a top surface of the first non-bump die 10N. In one embodiment, the first non-bump die 10N may be formed from a silicon-on-insulator (SOI) structure, which refers to a structure including a silicon substrate, a silicon layer, and a buried oxide layer sandwiched between the silicon substrate and the silicon layer. The first device layer 16 of the first non-bump die 10N may be formed by integrating electronic components (not shown) in or on the silicon layer of the SOI structure. The first dielectric layer 18 of the first non-bump die 10N may be the buried oxide layer of the SOI structure. The first silicon substrate 20 of the first non-bump die 10N may be the silicon substrate of the SOI structure. Herein, the first non-bump die 10N may have a thickness between 25 μm and 250 μm or between 10 μm and 750 μm, and the first silicon substrate 20 may have a thickness between 25 μm and 250 μm or between 10 μm and 750 μm, respectively. The first device layer 16 may have a thickness between 0.1 μm and 50 μm, and the first dielectric layer 18 may have a thickness between 10 nm and 2000 nm.

In another embodiment, the first non-bump die 10N may provide a microelectromechanical systems (MEMS) component (not shown), which is typically a switch and within the first device layer 16. Herein, the first non-bump die 10N may have a thickness between 25 μm and 300 μm or between 10 μm and 800 μm, and the first silicon substrate 20 may have a thickness between 25 μm and 300 μm or between 10 μm and 800 μm, respectively. The first device layer 16 may have a thickness between 0.5 μm and 100 μm, and may be formed from a combination of dielectric and metal layers (such as silicon oxide, silicon nitride, aluminum, titanium, copper, or the like). The first dielectric layer 18 may have a thickness between 10 nm and 10000 nm, and may be formed from silicon oxide, silicon nitride, aluminum oxide, or aluminum nitride.

The second non-bump die 12N includes a second device layer 22 and a second silicon substrate 24 over the second device layer 22. A bottom surface of the second device layer 22 is a bottom surface of the second non-bump die 12N, and a backside of the second silicon substrate 24 is a top surface of the second non-bump die 12N. If the first non-bump die 10N provides a MEMS component, the second non-bump die 12N may provide a complementary metal-oxide-semiconductor (CMOS) controller (not shown) that controls the MEMS component and is in the second device layer 22. Herein, the second non-bump die 12N has a thickness between 25 μm and 250 μm or between 10 μm and 750 μm, and the second silicon substrate 24 may have a thickness between 25 μm and 250 μm or between 10 μm and 750 μm, respectively. The second device layer 22 may have a thickness between 0.1 μm and 50 μm, and may be formed from a combination of dielectric and metal layers (such as silicon oxide, silicon nitride, aluminum, titanium, copper, or the like). In addition, if the first non-bump die 10N does not provide a MEMS component and is formed from a SOI structure, the second non-bump die 12N may be omitted. Further, the second non-bump die 12N may be shorter than the first non-bump die 10N.

Next, a first mold compound 26 is applied over the top surface of the carrier 14 to encapsulate the first non-bump die 10N and the second non-bump die 12N, as illustrated in FIG. 2. The first mold compound 26 may be an organic epoxy resin system or the like, which can be used as an etchant barrier to protect the first non-bump die 10N and the second non-bump die 12N against etching chemistries (details are described in following sections), such as potassium hydroxide (KOH), sodium hydroxide (NaOH), and acetylcholine (ACH). The first mold compound 26 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, or screen print encapsulation. In a typical compression molding, a molding pressure used for applying the first mold compound 26 is between 100 psi and 1000 psi. Since the first non-bump die 10N and the second non-bump die 12N are relatively thick and the bottom surfaces of the first non-bump die 10N and the second non-bump die 12N are essentially flat, there may be no vertical deformations occurring to the first non-bump die 10N and the second non-bump die 12N during this molding step.

A curing process (not shown) is then used to harden the first mold compound 26. The curing temperature is between 100° C. and 320° C. depending on which material is used as the first mold compound 26. A grinding process (not shown) is followed to provide a planarized top surface of the first mold compound 26. The carrier 14 is then removed to expose the bottom surface of the first device layer 16 of the first non-bump die 10N and the bottom surface of the second device layer 22 of the second non-bump die 12N, as shown in FIG. 3.

FIG. 4 shows a process step to provide a precursor package 28. Herein, a number of first die bumps 30 are formed underneath the first device layer 16 at the exposed bottom surface of the first device layer 16 to provide a first intact die 10. A number of second die bumps 32 are formed underneath the second device layer 22 at the exposed bottom surface of the second device layer 22 to provide a second intact die 12. The first die bumps 28 and the second die bumps 30 may be copper pillars, and are not covered by the first mold compound 26.

FIG. 5 provides a flow diagram that illustrates an exemplary process to provide a wafer-level fan-out (WLFO) package with enhanced performance from the precursor package 28 shown in FIG. 4. FIGS. 6-13 illustrate the steps associated with the process of FIG. 5. Although the flow diagram and the associated steps are illustrated in a series, they are not necessarily order dependent. Some steps may be done in a different order than that presented. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated in FIG. 5.

Firstly, a second mold compound 34 is formed underneath the first mold compound 26, as illustrated in FIG. 6 (step 100). The second mold compound 34 covers the exposed bottom surface of the first device layer 16 and the exposed bottom surface of the second device layer 22, and encapsulates each first die bump 30 and each second die bump 32. There is no air gap under the first intact die 10 and the second intact die 12. The second mold compound 34 may have a thickness between 5 μm and 200 μm. The second mold compound 34 and the first mold compound 26 may be formed from a same material or different materials. Since the second mold compound 34 directly connects to the first device layer 16 and the second device layer 22, the second mold compound 34 may be formed from a high thermal conductivity material for superior thermal performance of the first intact die 10 and the second intact die 12. The second mold compound 34 has a thermal conductivity greater than 2 W/m·K or greater than 10 W/m·K, such as thermoplastics or thermoset materials.

The second mold compound 34 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, and screen print encapsulation. In a typical compression molding, if the second mold compound 34 is formed of a high thermal conductivity material (>=2 W/m·K), a molding pressure and a temperature used for applying the second mold compound 34 is between 250 psi and 1000 psi, and between 100° C. and 350° C., respectively. Herein, since the precursor package 28 is relatively thick, there may be no vertical deformations occurring to the first intact die 10 and the second intact die 12 during this molding step.

A curing process (not shown) is then used to harden the second mold compound 34. The curing temperature is between 100° C. and 320° C. depending on which material is used as the second mold compound 34. A grinding process (not shown) is followed to provide a planarized bottom surface of the second mold compound 34. Next, the first mold compound 26 is thinned down to expose the backside of the first silicon substrate 20 of the first intact die 10 and a mold package 36 is provided, as illustrated in FIG. 7 (step 102). The thinning procedure may be done with a mechanical grinding process. Herein, the second intact die 12 has a lower height than the first intact die 10, such that the second silicon substrate 24 of the second intact die 12 is not exposed and the second intact die 12 is still fully encapsulated by the first mold compound 26.

After the backside of the first silicon substrate 20 is exposed, the first silicon substrate 20 is removed substantially to provide an etched package 38, as illustrated in FIG. 8 (step 104). The removal of the first silicon substrate 20 from the first intact die 10 provides a first thinned die 10T and forms a first opening 40 within the first mold compound 26 and over the first thinned die 10T. Herein, removing substantially a silicon substrate refers to removing at least 95% of the silicon substrate and leaving at most 2 μm of the silicon substrate. In desired cases, the first silicon substrate 20 is removed completely, such that the first dielectric layer 18 of the first thinned die 10T is exposed at a bottom of the first opening 40.

Removing substantially the first silicon substrate 20 may be provided by an etching process with a wet/dry etchant chemistry, which may be TMAH, KOH, ACH, NaOH, or the like. The first dielectric layer 18 may function as an etching stop to protect the first device layer 16 of the first thinned die 10T. The first mold compound 26 encapsulates and protects the second intact die 12 from the wet/dry etchant chemistry, and the second mold compound 34 protects the bottom surface of the first thinned die 10T and the bottom surface of the second intact die 12 from the etchant chemistry.

Next, a third mold compound 42 is applied to substantially fill the first opening 40, as illustrated in FIG. 9A (step 106). Herein, substantially filling an opening refers to filling at least 75% of the entire opening. The third mold compound 42 directly resides over the top surface of the first thinned die 10T. If there is no first silicon substrate 20 left in the first opening 40, the third mold compound 42 will directly reside over the first dielectric layer 18. In some cases, a portion of the third mold compound 42 may further reside over the first mold compound 26. The third mold compound 42 is separated from the second intact die 12 by the first mold compound 26. A top surface of the second intact die 12 is in contact with the first mold compound 26.

The third mold compound 42 may have a thermal conductivity greater than 2 W/m·K or greater than 10 W/m·K, and has an electrical resistivity greater than 1E6 Ohm-cm. In general, the higher the thermal conductivity of the third mold compound 42, the better the thermal performance of the first thinned die 10T. Further, the high electrical resistivity of the third mold compound 42 may improve the quality factor (Q) at high frequencies of the MEMS component provided in the first thinned die 10T, or may reduce signal loss in the first thinned die 10T if formed from an SOI structure.

The third mold compound 42 may be formed of thermoplastics or thermoset materials, such as PPS (poly phenyl sulfide), overmold epoxies doped with boron nitride or alumina thermal additives, or the like. The first, second, and third mold compounds 26, 34, and 42 may be formed of a same material or different materials. For instance, the second and third mold compounds 34 and 42 may be formed from a same material, while the first mold compound 26 may be formed from a different material. Unlike the third mold compound 42, the first mold compound 26 does not have thermal conductivity requirements.

The third mold compound 42 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, and screen print encapsulation. During the molding process of the third mold compound 42, liquefaction and molding pressure may not be uniform across the entire etched package 38. Because the combination of the first thinned die 10T and a portion of the second mold compound 34 directly underneath the first thinned die 10T are thinner than other portions of the etched package 38, they may suffer more deformations than the other portions of the etched package 38. In a typical compression molding, if the third mold compound 42 is formed of high thermal conductivity materials (>=2 W/m·K), a molding pressure and a temperature used for applying the second mold compound 42 is between 250 psi and 1000 psi, and between 100° C. and 350° C., respectively. A curing process (not shown) is then used to harden the third mold compound 42. The curing temperature is between 100° C. and 320° C. depending on which material is used as the third mold compound 42. A grinding process (not shown) is followed to provide a planarized top surface of the third mold compound 42.

Notice that the combination of the first thinned die 10T and the second mold compound portion 34 underneath the first thinned die 10T may have a thickness at least 8 μm. There is no air gap under the first thinned die 10T, and the bottom surface of the second mold compound 34 is planarized. As such, there may be no vertical deformations occurring to the first thinned die 10T during the molding step of the third mold compound 42. However, if there is no second mold compound 34 formed under the first mold compound 26, especially not filling gaps between the first die bumps 30 vertically below the first thinned die 10T, vertical deformations of the first thinned die 10T may occur during the molding step of the third mold compound 42. Without extra support in the gaps vertically below the first thinned die 10T, the first thinned die 10T may not endure high vertical molding pressure.

In some applications, the third mold compound 42 does not cover the top surface of the first mold compound 26. Instead, the top surface of the third mold compound 42 and the top surface of the first mold compound 26 are coplanar, as illustrated in FIG. 9B. Further, if a portion of the third mold compound 42 resides over the first mold compound 26, a grinding process may be applied to thin down the third mold compound 42 until the top surface of the third mold compound 42 and the top surface of the first mold compound 26 are coplanar.

In addition, if the top surface of the third mold compound 42 and the top surface of the first mold compound 26 are coplanar, there may be a fourth mold compound 44 applied over the first mold compound 26 and over the third mold compound 42, as illustrated in FIG. 9C. Herein, the fourth mold compound 44 may be formed of a same or different material as the first, second, and/or third mold compounds 26, 34, and/or 42. For instance, the first mold compound 26 and the fourth mold compound 44 may be formed of one material, while the second mold compound 34 and the third mold compound 42 may be formed of another material. Like the first mold compound 26, the fourth mold compound 44 does not have thermal conductivity requirements.

The fourth mold compound 44 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, lamination molding, and screen print encapsulation. A curing process (not shown) is then used to harden the fourth mold compound 44. The curing temperature is between 100° C. and 320° C. depending on which material is used as the fourth mold compound 44. A grinding process (not shown) is followed to provide a planarized top surface of the fourth mold compound 44.

After the third mold compound 42 is applied, the second mold compound 34 is thinned to expose each first die bump 30 and each second die bump 32, as illustrated in FIG. 10 (step 108). Herein, since the first die bumps 30 protrude from the bottom surface the first thinned die 10T and the second die bumps 32 protrude from the bottom surface the second intact die 12, the bottom surface of the first thinned die 10T and the bottom surface of the second intact die 12 are still fully covered by the second mold compound 34. The thinning procedure may be done with a mechanical grinding process.

With reference to FIGS. 11 through 13, a multilayer redistribution structure is formed (step 110) according to one embodiment of the present disclosure. Although the redistribution steps are illustrated in a series, the redistribution steps are not necessarily order dependent. Some steps may be done in a different order than that presented. Further, redistribution steps within the scope of this disclosure may include fewer or more steps than those illustrated in FIGS. 11-13.

A number of redistribution interconnections 46 are firstly formed underneath the second mold compound 34, as illustrated in FIG. 11. For the purpose of this illustration, the redistribution interconnections 46 include three first redistribution interconnections 46(1) and one second redistribution interconnection 46(2). In different applications, the redistribution interconnections 46 may include fewer or more first redistribution interconnections 46(1) and fewer or more second redistribution interconnections 46(2). Each first redistribution interconnection 46(1) is electrically coupled to a corresponding one of the first and second die bumps 30 and 32 exposed at the bottom surface of the second mold compound 34. The second redistribution interconnection 46(2) is configured to connect one first die bump 30 to a corresponding second die bump 32, such that the first thinned die 10T is connected to the second intact die 12. The connections between the redistribution interconnections 46 and the first/second die bumps 30/32 are solder-free.

Next, a dielectric pattern 48 is formed underneath the second mold compound 34 to partially encapsulate each first redistribution interconnection 46(1) as illustrated in FIG. 12. As such, a portion of each first redistribution interconnection 46(1) is exposed through the dielectric pattern 48. Further, the dielectric pattern 48 fully encapsulates the second redistribution interconnection 46(2). Herein, no portion of the second redistribution interconnection 46(2) is exposed through the dielectric pattern 48. In different applications, there may be extra redistribution interconnections (not shown) electrically coupled to the first redistribution interconnections 46(1) through the dielectric pattern 48, and extra dielectric patterns (not shown) formed underneath the dielectric pattern 48, such that a portion of each extra redistribution interconnection is exposed.

Lastly, a number of package contacts 50 are formed to complete a multilayer redistribution structure 52 and provide a WLFO package 54, as illustrated in FIG. 13. Each package contact 50 is formed at a bottom of the multilayer redistribution structure 52 and electrically coupled to an exposed portion of a corresponding first redistribution interconnection 46(1) through the dielectric pattern 48. Consequently, the first redistribution interconnects 46(1) connect the package contacts 50 to certain ones of the first and second die bumps 30 and 32. In addition, the package contacts 50 are separate from each other and extend underneath the dielectric pattern 48.

The multilayer redistribution structure 52 may be free of glass fiber or glass-free. Herein, the glass fiber refers to individual glass strands twisted to become a larger grouping. These glass strands may then be woven into a fabric. The dielectric pattern 48 may be formed of benzocyclobutene (BCB), polyimide, or other dielectric materials. The redistribution interconnections 46 may be formed of copper or other suitable metals. The package contacts 50 may be formed of at least one of copper, gold, nickel, and palladium. The multilayer redistribution structure 52 has a thickness between 2 μm and 300 μm.

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A method comprising: providing a mold package that includes a first intact die, a first mold compound, and a second mold compound, wherein: the first intact die comprises a first device layer, a first dielectric layer over the first device layer, a first silicon substrate over the first dielectric layer, and a plurality of first die bumps underneath the first device layer; the first mold compound resides around the first intact die to encapsulate sidewalls of the first intact die, wherein a backside of the first silicon substrate is exposed; and the second mold compound, which is different from the first mold compound, is formed underneath the first mold compound to cover a bottom surface of the first device layer, wherein a combination of the first device layer and the second mold compound fully encapsulates each of the plurality of first die bumps; removing substantially the first silicon substrate of the first intact die to provide a first thinned die and form an opening within the first mold compound and over the first thinned die, wherein the first thinned die has a top surface exposed at a bottom of the opening; applying a third mold compound to substantially fill the opening and directly contact the top surface of the first thinned die; and thinning the second mold compound to expose each of the plurality of first die bumps after applying the third mold compound.
 2. The method of claim 1 wherein the first intact die provides a microelectromechanical systems (MEMS) component.
 3. The method of claim 1 wherein the first intact die is formed from a silicon-on-insulator (SOI) structure, wherein the first device layer of the first intact die is formed from a silicon layer of the SOI structure, the first dielectric layer of the first intact die is a buried oxide layer of the SOI structure, and the first silicon substrate of the first intact die is a silicon substrate of the SOI structure.
 4. The method of claim 1 wherein the mold package further comprises a second intact die, which includes a second device layer, a second silicon substrate over the second device layer, and a plurality of second die bumps underneath the second device layer, wherein: the first intact die is taller than the second intact die; the first mold compound encapsulates sidewalls and a top surface of the second intact die; and the second mold compound covers a bottom surface of the second device layer and encapsulates each of the plurality of second die bumps.
 5. The method of claim 4 wherein the first intact die provides a MEMS component and the second intact die provides a complementary metal-oxide-semiconductor (CMOS) controller that controls the MEMS component.
 6. The method of claim 1 wherein the third mold compound has an electrical resistivity greater that 1E6 Ohm-cm.
 7. The method of claim 1 wherein the third mold compound has a thermal conductivity greater than 2 W/m·K.
 8. The method of claim 1 wherein the third mold compound has a thermal conductivity greater than 10 W/m·K.
 9. The method of claim 1 wherein the first mold compound, the second mold compound, and the third mold compound are formed from different materials.
 10. The method of claim 1 wherein the second mold compound and the third mold compound are formed from a same material.
 11. The method of claim 1 wherein the second mold compound has a thickness between 5 μm and 200 μm before the thinning process of the second mold compound.
 12. The method of claim 1 wherein the top surface of the first thinned die exposed at the bottom of the opening is a top surface of the first dielectric layer.
 13. The method of claim 1 further comprising forming a multilayer redistribution structure underneath the second mold compound, wherein the multilayer redistribution structure comprises a plurality of package contacts at a bottom of the multilayer redistribution structure and redistribution interconnections that connect the plurality of package contacts to certain ones of the plurality of first die bumps.
 14. The method of claim 13 wherein: the multilayer redistribution structure is glass-free and connections between the redistribution interconnections and each of the plurality of first die bumps are solder-free.
 15. The method of claim 1 wherein providing the mold package comprises: providing a precursor package that includes the first intact die and the first mold compound, wherein the first mold compound encapsulates the sidewalls and a top surface of the first intact die, while the bottom surface of the first device layer and each of the plurality of first die bumps are exposed; applying the second mold compound underneath the first mold compound to cover the bottom surface of the first device layer, wherein the combination of the first device layer and the second mold compound fully encapsulates each of the plurality of first die bumps; and thinning down the first mold compound to expose the backside of the first silicon substrate after applying the second mold compound.
 16. The method of claim 15 wherein providing the precursor package comprises: attaching a first non-bump die to a top surface of a carrier, wherein the first non-bump die includes the first device layer, the first dielectric layer over the first device layer, and the first silicon substrate over the first dielectric layer; applying the first mold compound over the top surface of the carrier to encapsulate the first non-bump die; removing the carrier to expose the bottom surface of the first device layer of the first non-bump die; forming the plurality of first die bumps at the exposed bottom surface of the first device layer to complete the first intact die, wherein the plurality of first die bumps are not covered by the first mold compound.
 17. The method of claim 1 wherein a portion of the third mold compound resides over the first mold compound.
 18. The method of claim 1 wherein a top surface of the first mold compound and a top surface of the third mold compound are coplanar.
 19. The method of claim 18 further comprising applying a fourth mold compound over the top surface of the first mold compound and the top surface of the third mold compound.
 20. The method of claim 19 wherein the first mold compound and the fourth mold compound are formed from a same material.
 21. The method of claim 1 wherein the third mold compound is applied by a compression molding process.
 22. The method of claim 1 wherein a molding pressure used for applying the third mold compound is lower than 1000 psi. 